In the field of blood transfusion, in place of so-called whole blood transfusion comprising transfusing a whole blood product obtained by adding an anti-coagulant to blood collected from a blood donor, so-called component transfusion comprising separating a necessary blood component from the whole blood product and transfusing that blood component to a blood recipient is generally carried out. The component transfusion includes erythrocyte transfusion, platelet transfusion, plasma transfusion, etc., depending on the blood component necessary for a blood recipient. Blood component preparations used in these blood transfusions include red cell products, platelet products, plasma preparations, etc. In recent years, a so-called leukocyte-free blood transfusion has been developed in which a blood product is transfused after being freed of leukocytes contained therein as contaminant. Leukocyte-free blood transfusion was developed after revelation that adverse side effects accompanying blood transfusion, for example, relatively slight adverse side effects such as headache, nausea, chill, non-hemolytic pyretic reaction, etc., and serious adverse side effects such as allo-antigen sensitization, viral infection, post-transfusion GPHD, etc., which have serious influences on a blood recipient, are caused mainly by leukocytes contained as contaminants in a blood product used in transfusion.
To prevent relatively slight adverse side effects such as headache, nausea, chill, pyrexia, etc., it is sufficient that leukocytes in a blood product be removed until the remaining rate becomes less than 10.sup.-1 -10.sup.-2. In addition, it is said that to prevent serious adverse side effects such as alloantigen sensitization, viral infection, etc., it is sufficient that leukocytes be removed until the remaining rate becomes less than 10.sup.-4 -10.sup.-6.
Methods for removing leukocytes from a blood product are divided broadly into two categories, i.e., the centrifugation method in which leukocytes are separated and removed with a centrifuge by utilizing the difference in specific gravity among blood components, and the filter method in which leukocytes are removed by using a filter medium composed of a fibrous material or a porous element such as a porous material having interconnecting voids. The filter method is being generalized at present because it has advantages such as excellent leukocyte-removing capability, easy operations, and low cost. Of the filter method, a method of removing leukocytes by adhesion or adsorption by using non-woven fabric as a filter medium is the most widespread at present because of its especially excellent leukocyte-removing capability.
As to the mechanism of the removal of leukocytes with a filter apparatus using the above-mentioned fibrous material or porous material, removal is caused mainly because leukocytes brought into contact with the filter medium surface are adhered to or adsorbed on the filter medium surface. For example, EP-A-0155003 discloses a technique using non-woven fabric as a filter medium. In addition, WO93/01880 discloses a leukocyte-removing filter medium produced by dispersing, in a dispersion medium, a mass of a large number of small fiber pieces having a fiber diameter of 0.01 .mu.m or less and a length of approximately 1-50 .mu.m, and short fibers which have a fineness of approximately 0.05-0.75 d and an average length of 3-15 mm which can be spun and woven, and then removing the dispersion medium from the resulting dispersion.
Existing leukocyte-removing filters have a leukocyte-removing capability such that the number of remaining leukocytes is 1.times.10.sup.5 or less. Under such circumstances, two important requirements have been imposed on leukocyte-removing filters in the market.
The first requirement is to improve the recovery of useful components and improve ease of handling by making unnecessary a procedure for recovering useful components remaining in a filter and a tube because of the presence of physiological saline and air. Improving the recovery of the useful components as compared with the existing leukocyte-removing filters is very worthwhile because blood as a starting material for blood products is often precious blood provided by well-intentioned blood donation, and unrecoverable blood remaining in a leukocyte-removing filter is wasted and discarded as it is together with the filter. It is, however, difficult to greatly improve the recovery of the useful components in a leukocyte-removing filter obtained according to prior art.
The second requirement is to completely prevent serious adverse side effects caused by leukocytes transfused into a patient, by attaining a leukocyte removal rate higher than that of the existing leukocyte-removing filters. However, in the case of a leukocyte-removing filter obtained according to prior art, it is difficult to attain such a high leukocyte removal rate so that adverse side effects can be completely prevented.
In order to satisfy the above requirements set out by the market, the present inventors earnestly investigated and consequently have accomplished the preceding invention (WO97/23266). The preceding invention is explained below in detail. Objects of the preceding invention are to provide leukocyte-removing filter medium which has a much higher leukocyte-removing capability per unit volume than do conventional filter media, and to permit satisfactory flow of a leukocyte-containing fluid; to provide a process for producing the filter medium; and to provide a filter apparatus containing the filter medium, and a method for removing leukocytes from a leukocyte-containing fluid by using the filter apparatus. The filter medium of the preceding invention is a leukocyte-removing filter medium comprising a porous element having pores with an average pore size of less than 100 .mu.m and not less than 1.0 .mu.m, and a fiber structure held thereby and composed of a plurality of fibers having an average fiber diameter of less than 1.0 .mu.m and not less than 0.01 .mu.m, wherein the void content of the filter medium is less than 95% and not less than 50%, the percentage of the fiber structure relative to the filter medium (this percentage is hereinafter referred to as holding amount) is less than 30 wt % and not less than 0.01 wt %. The ratio of the average pore size of pores of the porous element (hereinafter referred to also as the average pore size of the porous element) to the average fiber diameter of the fibers constituting the fiber structure (hereinafter referred to also as the average fiber diameter of the fiber structure) is less than 2000 and not less than 2, and the fiber structure forms a network structure.
In addition, the process for producing the leukocyte-removing filter medium of the preceding invention is, for example, a process of dispersing, in a solvent, fibers with an average fiber diameter of less than 1.0 .mu.m and not less than 0.01 .mu.m obtained by splitting splittable fiber, and making the resulting dispersion into paper together with a porous element having pores with an average pore size of less than 100 .mu.m and not less than 1.0 .mu.m (hereinafter referred to as a porous element having an average pore size of less than 100 .mu.m and not less than 1.0 .mu.m), to hold the fibers in the porous element.
Furthermore, in the preceding invention, a leukocyte-removing filter apparatus was found which contains a leukocyte-removing filter medium properly located therein which comprises a porous element having an average pore size of less than 100 .mu.m and not less than 1.0 .mu.m and a fiber structure composed of a plurality of fibers with an average fiber diameter of less than 1.0 .mu.m and not less than 0.01 .mu.m (hereinafter referred to as a fiber structure having an average fiber diameter of less than 1.0 .mu.m and not less than 0.01 .mu.m). The fiber structure is held by the porous element, wherein the void content of the filter medium is less than 95% and not less than 50%. The holding amount of the fiber structure relative to the filter medium is less than 30 wt % and not less than 0.01 wt %, the ratio of the average pore size of the porous element to the average fiber diameter of the fiber structure is less than 2000 and not less than 2, and the fiber structure forms a network structure. A method for removing leukocytes from a leukocyte-containing fluid by filtering the leukocyte-containing fluid by using the apparatus above was also found.
A typical filter medium of the preceding invention is shown in FIG. 1A and FIG. 1B.
FIG. 1A is an electron micrograph of the surface of the filter medium having a curvilinear network structure of the preceding invention.
FIG. 1B is an electron micrograph of a section of the filter medium shown in FIG. 1A.
The term "average fiber diameter" in the preceding invention means a value obtained by taking a scanning electron micrograph of the fibers constituting the fiber structure, measuring the diameters of 100 or more fibers randomly selected therefrom, and calculating the number average of these diameters. The average fiber diameter may be measured either before holding the fibers in the porous element used as a matrix, or after holding the fibers in the porous element used as a matrix. Particularly, when the porous element is composed of an assembly of fibers, the average fiber diameter is preferably measured before holding the fibers in the porous element because the measurement can be more accurately carried out.
Fibers having an average fiber diameter of less than 0.01 .mu.m are not suitable because they are so poor in strength that they tend to be cut by leukocytes or other hemocyte components which collide with the fibers during the treatment of a leukocyte-containing fluid. Fibers having an average fiber diameter of 1.0 .mu.m or more are not suitable because the porosity of the filter medium is decreased, resulting in an unsatisfactory flow of a leukocyte-containing fluid. For capturing, for example, poorly adherent lymphocytes having a relatively small diameter among leukocytes by efficiently bringing them into contact with the filter medium at many points, the average fiber diameter of the fibers is preferably less than 0.8 .mu.m and not less than 0.01 .mu.m.
In the fiber structure according to the preceding invention, fibers having a very small average fiber diameter form a network structure. Such a reticulate fiber structure is held by the porous element. In the preceding invention, the passage "the fiber structure is held by the porous element" means that as shown in FIG. 1A, the above-mentioned reticulate fiber structure is fixed in the matrix so that they may cover the pore portions of the porous element used as matrix. FIG. 1A shows an electron micrograph of the filter medium having a typical network structure of the preceding invention. Physical and structural characteristics of the filter medium of the preceding invention are described below with reference to FIG. 1A.
In the filter medium of the preceding invention, a plurality of fibers having an average fiber diameter of less than 1.0 .mu.m and not less than 0.01 .mu.m constitute the fiber structure by forming a network structure, and the fiber structure is held by the porous element having pores with an average pore size of less than 100 .mu.m and not less than 1.0 .mu.m. The fibers constituting the fiber structure, however, are not in the form of a bundle but are so-called single fibers, each of which is in a split state. A plurality of such single fibers are physically entangled with one another to form the network structure. As the network structure according to the preceding invention, a structure is exemplified in which, as represented by the network structure shown in FIG. 1A, fibers constituting the fiber structure have a curved structure, so that meshes formed by them are curvilinear.
The reticulate fiber structure is preferably uniformly held by the porous element in a section perpendicular to the flow of a leukocyte-containing fluid because leukocytes can be efficiently captured. The uniform holding of the fiber structure by the porous element in a section perpendicular to the flow of a leukocyte-containing fluid means that the introducing amount (density) of the fiber structure is substantially the same in various portions of the filter medium which are randomly sampled in the section perpendicular to the flow of the leukocyte-containing fluid. In practice, this introducing amount can be determined by measuring the variation of the amount of the fiber structure present in a definite amount of the filter medium in the sampled portions of the filter medium.
The following is especially preferable: the introducing amount of the fiber structure is substantially the same in various portions of the filter medium which are randomly sampled in a section perpendicular to the flow of a leukocyte-containing fluid, and moreover, the mesh size distribution is substantially the same in the various portions, so that substantially the same reticulate structures are formed therein. Such a state is expressed by the passage "a uniform network structure is formed" in the preceding specification. More specifically, the passage "a uniform network structure is formed" means a state at which, when observed by an electron microscope, the network structures in the randomly sampled portions of the filter medium are similar in mesh size distribution and mesh shape, and are regarded as substantially the same. A state at which no uniform network structure has been formed means a state at which when the network structures in the randomly sampled portions of the filter medium are observed, it is possible to judge that the mesh size distribution is widely different in these portions and that the shape of mesh is also clearly different in these portions.
In the filter medium of the preceding invention, the following are preferable: a fiber structure having an average fiber diameter of less than 1.0 .mu.m and not less than 0.01 .mu.m which forms a network structure and is held by a porous element having an average pore size of less than 100 .mu.m and not less than 1.0 .mu.m, the void content of the filter medium is less than 95% and not less than 50%, and the ratio of the average pore size of the porous element to the average fiber diameter of the fiber structure is less than 2000 and not less than 2.
Here, the average pore size is a value obtained by measurement using a mercury injection method. That is, when the amount of mercury injected is taken as 0% at a mercury injection pressure of 1 psia and as 100% at a mercury injection pressure of 2650 psia, a pore size corresponding to an amount of mercury injected of 50% was taken as the average pore size. An average pore size of less than 1.0 .mu.m is not suitable because a leukocyte-containing fluid does not flow, and an average pore size of 100 .mu.m or more is not suitable because maintenance of the fiber structure often becomes difficult.
For keeping good flow of a leukocyte-containing fluid, the ratio of the average pore size of the porous element to the average fiber diameter of the fiber structure is preferably less than 2000 and not less than 2. A ratio of the average pore size of the porous element to the average fiber diameter of the fiber structure of less than 2 is not suitable because there is almost no difference between the pore size of the porous element and the diameter of fibers constituting the fiber structure, so that the pores of the porous element are blocked with the fibers, resulting in an extremely bad flow of a leukocyte-containing fluid. A ratio of the average pore size of the porous element to the average fiber diameter of the fiber structure of 2000 or more is not suitable because the pore size of the porous element is so large that it becomes difficult to hold the fiber structure in the porous element so that the fiber structure may cover the pores of the porous element, and therefore the leukocyte-removing capability is extremely deteriorated and, moreover, the entanglement of the fiber structure and the porous element with each other becomes insufficient, so that the fiber structure tends to be released. The ratio of the average pore size of the porous element to the average fiber diameter of the fiber structure is more preferably less than 1,800 and not less than 10.
The porous element according to the preceding invention includes fiber assemblies, porous membranes, spongy and connectedly porous materials, etc., which have an average pore size of less than 100 .mu.m and not less than 1 .mu.m. As the porous element, fiber assemblies composed of long fibers are preferable. Preferable examples of the form of the fiber assemblies are non-woven fabric, woven fabric, and knitted fabric. Non-woven fabric is especially preferable. When the porous element is a fiber assembly, the ratio of the average fiber diameter of the fiber assembly to the average fiber diameter of the fiber structure is particularly preferable to be less than 1,000 and not less than 10 for keeping good flow of a leukocyte-containing fluid. As a material for the porous element, any material capable of forming a non-woven fabric, woven fabric, knitted fabric, porous membrane, spongy and connectedly porous material or the like, such as polyurethanes, polyesters, polyolefins, polyamides, polystyrenes, polyacrylonitriles, cellulose, cellulose acetate, etc. can be used.
Furthermore, in the leukocyte-removing filter medium of the preceding invention, the holding amount of the fiber structure relative to the filter medium is preferably less than 30 wt % and not less than 0.01 wt %. A holding amount of less than 0.01 wt % is not suitable because a fiber content sufficient to capture leukocytes in a leukocyte-containing fluid cannot be attained. A holding amount of 30 wt % or more is not suitable because the content of fibers introduced into the porous element is too high, and the pore portions of the porous element are blocked, so that a leukocyte-containing fluid does not flow. The holding amount of the fiber structure relative to the filter medium is more preferably less than 10 wt % and not less than 0.03 wt %.
The holding amount can be measured on the basis of the difference of the weight before and after holding the fiber structure in the porous element. When the holding amount of the fiber structure is as small as about 3 wt % per unit weight of the filter medium, a method comprising dissolving only the fiber structure, extracting its component, and determining the amount of the extracted component can be employed for determining the holding amount of the fiber structure with a precision higher than that of the weight measurement. This quantitation method is concretely explained below by taking the case where the fiber structure is made of cellulose. The filter medium is immersed in a solution containing cellulase dissolved therein, and is shaken to decompose the cellulose of the fiber structure into glucose, which is extracted. The extracted glucose is quantitated by using a commercially available glucose-quantitating reagent, and, the amount of the fiber structure held by the porous element is calculated from the amount of glucose quantitated.
For attaining a high leukocyte-removing capability, the fiber structure is preferably held by the whole porous element. The fiber structure, however, may be supported on one surface of the porous element if it is difficult to hold the fiber structure in the porous element so that the fiber structure may be present also in the innermost of the porous element, owing to a restriction by a production process. In such a case, as a means for easily improving the leukocyte-removing capability of the filter medium by increasing the holding amount of the fiber structure, it is also possible to support the fiber structure on each of the two surfaces of the porous element. In both cases, substantially supporting the fiber structure uniformly on the porous element is preferable for attaining a high leukocyte-removing capability.
In the leukocyte-removing filter medium of the preceding invention, the void content is preferably less than 95% and not less than 50%. A void content of the filter medium of less than 50% is not suitable because the flow of a leukocyte-containing fluid is not good. A void content of 95% or more is not suitable because the mechanical strength of the filter medium is so low that the filter medium is crushed during the treatment of a leukocyte-containing fluid, and no longer fulfils its function as a filter medium.
The void content is measured by measuring the dry weight (W.sub.1) of a piece with a predetermined area obtained by cutting the filter medium, and measuring also the thickness of the piece, followed by calculation of the volume (V) of the piece. The piece of the filter medium is immersed in pure water and deaerated, after which the weight (W.sub.2) of the water-containing piece of the filter medium is measured. From these values, the void content is calculated by the following equation. In the following equation, .rho. is the density of pure water. EQU Void content (%)=(W.sub.2 -W.sub.1).times..rho..times.100/V
The thickness of the leukocyte-removing filter medium of the preceding invention is preferably less than 30 mm and not less than 0.1 mm in the direction of flow of a leukocyte-containing fluid. A thickness of less than 0.1 mm is not desirable because the frequency of collision between leukocytes in the leukocyte-containing fluid and the filter medium is decreased, so that it is difficult to attain a high leukocyte-removing capability. A thickness of 30 mm or more is not desirable, for example, because the resistance to passage of the leukocyte-containing fluid is increased, resulting in elongation of the treatment time and hemolysis accompanying the breakage of erythrocyte membranes. The thickness of the filter medium in the direction of flow is more preferably less than 15 mm and not less than 0.1 mm.
When adopting a production process to obtain the filter medium of the preceding invention wherein the process is characterized by dispersing fibers with an average fiber diameter of less than 1.0 .mu.m and not less than 0.01 .mu.m in a solvent, and then making the resulting dispersion into paper together with a porous element with an average pore size of less than 100 .mu.m and not less than 1.0 .mu.m to hold the fibers in the porous element, the ratio of the average pore size of the porous element to the average fiber diameter of the fiber structure is more preferably less than 300 and not less than 16. In addition, the holding amount of the fiber structure relative to the filter medium is preferably less than 5.0 wt % and not less than 0.3 wt %. In addition, the average fiber diameter of the fiber structure is more preferably less than 0.5 .mu.m and not less than 0.05 .mu.m.
When the leukocyte-removing filter medium of the preceding invention is treated with a binder such as a water-insoluble polymer solution in post-processing, the network structure is generally liable to be destroyed. For example, fibers constituting the fiber structure are bound into a bundle, or a film-like substance is formed among a plurality of the fibers. Therefore, it is preferable to avoid treatment with such a binder. On the other hand, when the fibers are relatively thick and short and are not sufficiently physically entangled with the porous element, treating the filter medium with a binder such as a relatively dilute water-insoluble polymer solution in post-processing is preferable because it permits effective fixation of the fibers in the porous element and prevention of the release of the fibers.
The following, for example, is also possible: the surface of the leukocyte-removing filter medium of the preceding invention is modified into a surface to which platelets or erythrocytes hardly adhere, whereby the recovery of platelets or erythrocytes is improved and only leukocytes are removed. As a method for modifying the surface of the filter medium, there are mentioned surface graft polymerization, coating with a polymeric material, electron discharge machining, etc.
As a polymeric material used for modifying the surface of the filter medium by the surface graft polymerization or the coating with a polymeric material, polymeric materials having one or more nonionic hydrophilic groups are preferable. The nonionic hydrophilic groups include hydroxyl group, amide group, poly-(ethylene oxide) chains, etc. Monomers usable for synthesizing the polymeric material having one or more nonionic hydrophilic groups include, for example, 2-hydroxyethyl methacrylate, 2-hydroxyethyl acrylate, vinyl alcohol (prepared by hydrolyzing a polymer obtained by polymerizing vinyl acetate), methacrylamide and N-vinylpyrrolidone. Of the monomers mentioned above, 2-hydroxyethyl methacrylate and 2-hydroxyethyl acrylate are preferable from the viewpoint of easy availability, ease of handling in polymerization, treating capability for a leukocyte-containing fluid, etc.
The polymeric material used for the above-mentioned surface graft polymerization or coating with the polymeric material is preferably a copolymer containing 0.1 to 20 mole % of monomer units derived from a polymerizable monomer having one or more nonionic hydrophilic groups and/or basic nitrogen-containing functional groups. The basic nitrogen-containing functional groups include primary amino group, secondary amino group, tertiary amino group, quaternary amino group, and nitrogen-containing aromatic ring groups such as pyridyl group, imidazole group, etc. The polymerizable monomer having one or more basic nitrogen-containing functional groups includes methacrylic acid derivatives such as dimethylaminoethyl methacrylate, diethylaminoethyl methacrylate, dimethylaminopropyl methacrylate, 3-dimethylamino-2-hydroxypropyl methacrylate, etc.; allylamine; vinyl derivatives of nitrogen-containing aromatic compounds, such as p-vinylpyridine, 4-vinylimidazole, etc.; and quaternary ammonium salts obtained by reacting the above-exemplified vinyl compound with an alkyl halide. Of the polymerizable monomers mentioned above, dimethylaminoethyl methacrylate and diethylaminoethyl methacrylate are preferable from the viewpoint of easy availability, ease of handling in polymerization, treating capability for a leukocyte-containing fluid, etc.
When the content of monomer units derived from the polymerizable monomer having one or more basic nitrogen-containing functional groups in the copolymer obtained is less than 0.1%, no sufficient inhibitory effect on the adhesion of platelets to the surface of the filter medium is undesirably obtained. When the content of monomer units derived from the polymerizable monomer having one or more basic nitrogen-containing functional groups in the copolymer is more than 20%, not only leukocytes but also useful components such as platelets and erythrocytes are undesirably apt to adhere to the surface of the filter medium. The content of monomer units derived from the polymerizable monomer having one or more basic nitrogen-containing functional groups in the copolymer is more preferably 0.2 to 5%.
In the preceding invention, as a result of earnest investigations for providing a process for producing a leukocyte-removing filter medium, it was found that by dispersing fibers with an average fiber diameter of less than 1.0 .mu.m and not less than 0.01 .mu.m in a dispersion medium and making the resulting dispersion into paper together with a porous element having an average pore size of less than 100 .mu.m and not less than 1.0 .mu.m to hold the fibers in the porous element, a leukocyte-removing filter medium can be produced which comprises the porous element and a fiber structure composed of a plurality of the fibers, wherein the void content of the filter medium is less than 95% and not less than 50%, the holding amount of the fiber structure relative to the filter medium is less than 30 wt % and not less than 0.01 wt %, the ratio of the average pore size of the porous element to the average fiber diameter of the fiber structure is less than 2000 and not less than 2, and wherein the fiber structure forms a network structure.
For the formation of the network structure by the fiber structure in the filter medium of the preceding invention, the fibers having an average fiber diameter of less than 1.0 .mu.m and not less than 0.01 .mu.m should have, for example, the following properties: they have a curved shape, are per se flexible and easy to curve, and are relatively short. Fibers having no curved shape in themselves are also suitable after being curved by heat treatment, mechanical treatment or treatment with any of various chemicals.
The above-mentioned fibers having an average fiber diameter of less than 1.0 .mu.m and not less than 0.01 .mu.m can be produced, for example, by subjecting the following fiber to physical stirring with a mixer or the like, treatment with a high-pressure liquid jet, treatment using a high-pressure homogenizer, or the like: splittable fiber represented by regenerated cellulose fiber or microporous splittable acrylic fiber, or splittable conjugate fiber obtained by any of the well-known methods disclosed in JP-B-47-37648, JP-A-50-5650, JP-A-53-38709, etc.
As a material for the fiber which is easy to curve, cellulose, polyacrylonitriles, polyesters, polyolefins, polyamides, etc. are suitable, though there can be used any material so long as when it is processed into fiber having an average fiber diameter of less than 1.0 .mu.m and not less than 0.01 .mu.m, the fiber can be curved by heat treatment or mechanical treatment.
A process for obtaining the fibers having the above-mentioned specific average fiber diameter by subjecting regenerated cellulose fiber among the splittable fibers mentioned above to acid treatment or alkali treatment if necessary, and then physically stirring the thus treated fiber by the use of a mixer or the like to fibrillate the same, is especially preferable because fibers having a very small fiber diameter and a curved shape can easily be obtained to easily form a network structure. This process for obtaining the fibers having an average fiber diameter of less than 1.0 .mu.m and not less than 0.01 .mu.m by fibrillating regenerated cellulose fiber is concretely explained below in further detail. First, commercially available regenerated cellulose fiber having a fiber diameter of about 10 .mu.m is cut to a predetermined length, immersed in an about 3 wt % aqueous sulfuric acid solution, and subjected to acid treatment therein with mild stirring at 70.degree. C. for 30 minutes. When the regenerated cellulose fiber subjected to the acid treatment is washed with water, and then vigorously stirred in water with a mixer at 10,000 rpm for 30 to 90 minutes, the regenerated cellulose fiber is fibrillated to be decreased in diameter, and the desired fibers can be finally obtained.
Fibers with an average fiber diameter of less than 1.0 .mu.m and not less than 0.01 .mu.m obtained by using well-known archipelago-type fiber as a starting material can also have a curved shape and are suitable for producing the above-mentioned leukocyte-removing filter medium. These fibers are produced by forming the starting fiber into a curved shape by previous heat treatment or mechanical treatment if necessary, and then dissolving away the sea component by the use of any of various solvents.
The thus obtained fibers having an average fiber diameter of less than 1.0 .mu.m and not less than 0.01 .mu.m are dispersed in a dispersion medium to a concentration of about 0.01 g/L to about 1 g/L to obtain a fiber dispersion. Pure water, aqueous solutions containing 0.1%-5% of a surfactant, and aqueous solutions which have been increased in viscosity by the addition of approximately 0.1%-5% of a polyacrylamide in order to further improve the dispersibility of the fibers have been used as the dispersion medium.
Next, the bottom of a funnel-shaped vessel is covered with a porous element having an average pore size of less than 100 .mu.m and not less than 1.0 .mu.m, and the aforesaid fiber dispersion is poured into the vessel and once accumulated, water is discharged at a stretch, and then the porous element is dried, whereby the filter medium can be obtained. In this case, the fibers are preferably short because they can be held in the porous element so that they may be present also in the innermost part of the porous element.
Treating the filter medium produced by the above production process with a high-pressure liquid jet at about 3 kg/cm.sup.2 to 200 kg/cm.sup.2 is preferable because the treatment makes it possible to hold the fibers in the porous element more uniformly so that they may be present also in the innermost part in the direction of thickness of the porous element.
The next object of the preceding invention is to provide a leukocyte-removing filter apparatus which removes leukocytes from a leukocyte-containing fluid while minimizing loss of useful blood components, and which can attain a high leukocyte removal rate, and a method for removing leukocytes using the apparatus; and to provide a leukocyte-removing filter apparatus which can attain a leukocyte removal rate much higher than that of conventional leukocyte-removing filter apparatuses, and a method for removing leukocytes using this apparatus. The present inventors earnestly investigated and consequently found that the above object can be achieved by filtering a leukocyte-containing fluid by the use of a filter apparatus obtained by properly locating the filter medium of the preceding invention in a container at least having an inlet and an outlet.
The leukocyte-removing filter apparatus of the preceding invention is an apparatus obtained by properly locating a filter comprising the filter medium of the preceding invention in a container at least having an inlet and an outlet. Either a sheet or a laminate of two or more sheets of the filter medium may be packed in the container in the direction of flow of a leukocyte-containing fluid. On the other hand, for example, when a solution containing a polymeric material is poured over the filter medium to carry out coating treatment for the purpose of modifying the surface of the filter medium, the lowest sheet of the filter medium in the apparatus of the preceding invention adheres to the inner surface of the container to cause a one-sided flow of a leukocyte-containing fluid in some cases. In such a case, the one-sided flow of the leukocyte-containing fluid caused by the adhesion of the filter medium to the inner surface of the container can be prevented by inserting a relatively coarse filter medium as the lowest layer.
The leukocyte-removing filter apparatus of the preceding invention may further contain other filter media upstream and/or downstream to the filter medium of the preceding invention.
In general, a leukocyte-containing fluid often contains very small aggregates. It is also possible to use a prefilter in order to remove leukocytes from such a leukocyte-containing fluid containing very small aggregates. As the prefilter, there are preferably used, for example, an assembly of fibers having an average fiber diameter of 8 .mu.m to 50 .mu.m and a connectedly porous material having pores with an average pore size of 20 .mu.m to 200 .mu.m.
In the preceding invention, the filter medium of the leukocyte-removing filter apparatus preferably has a sectional area in a direction normal to the direction of flow of a leukocyte-containing fluid of less than 100 cm.sup.2 and not less than 3 cm.sup.2. When the sectional area is less than 3 cm.sup.2, the flow of the leukocyte-containing fluid is extremely restricted, and therefore such a small sectional area, is not desirable. When the sectional area is 100 cm.sup.2 or more, the filter would be thinned unavoidably, so that high leukocyte-removing capability can not be attained when the filter apparatus is increased in size. Therefore, such a large sectional area also is not desirable.
The method for removing leukocytes of the preceding invention comprises treating a leukocyte-containing fluid by the use of the leukocyte-removing filter apparatus of the preceding invention, and recovering the filtrate. In detail, it is a method for removing leukocytes from a leukocyte-containing fluid which comprises using an apparatus comprising 1) an inlet, 2) a filter comprising the filter medium of the preceding invention, and 3) an outlet, introducing the leukocyte-containing fluid through the inlet, and recovering the filtrate obtained by filtration through the filter medium, through the outlet.
The leukocyte-containing fluid to be filtered by the use of the leukocyte-removing filter apparatus of the preceding invention includes, for example, whole blood products, concentrated red cell products and platelet concentrates, as well as body fluids.
When the leukocyte-containing fluid is a whole blood product or a concentrated red cell product, the leukocyte-containing fluid is preferably treated by the use of a leukocyte-removing filter apparatus having an apparatus capacity per unit of less than 20 mL and not less than 3 mL. The term "unit" used here means approximately 300 mL-550 mL of the whole-blood product or the concentrated red cell product. When the apparatus capacity per unit is less than 3 mL, there is a strong, undesirable possibility that high leukocyte removal rate can not be attained. When the apparatus capacity per unit is 20 mL or more, the amount of unrecoverable useful components in the leukocyte-containing fluid which remain inside the apparatus is undesirably increased. By filtering a whole blood product or a concentrated red cell product by the use of the leukocyte-removing filter apparatus of the preceding invention, leukocytes can be removed to such an extent that the number of remaining leukocytes in the recovered fluid is less than 1.times.10.sup.3 /unit.
When the leukocyte-containing fluid is a platelet concentrate, the leukocyte-containing fluid is preferably treated by the use of a leukocyte-removing filter apparatus having an apparatus capacity per 5 units of less than 10 mL and not less than 1 mL. The term "5 units" used here means about 170 mL to about 200 mL of the concentrated platelet preparation. When the apparatus capacity per 5 units is less than 1 mL, there is undesirably a strong possibility that high leukocyte removal rate can not be attained. When the apparatus capacity per 5 units is 10 mL or more, the amount of unrecoverable useful components remaining inside the apparatus is undesirably increased. By filtering a platelet concentrate by the use of the leukocyte-removing filter apparatus of the preceding invention, leukocytes can be removed to such an extent that the number of remaining leukocytes in the recovered fluid is less than 1.times.10.sup.3 /5 units.
When leukocytes are removed simultaneously with blood transfusion at a bedside in a hospital by using the leukocyte-removing filter apparatus of the preceding invention, a leukocyte-containing fluid is preferably filtered at a rate of less than 20 g/min. and not less than 1 g/min. On the other hand, when leukocytes are removed from a blood product for transfusion in a Blood Center by the use of the leukocyte-removing filter apparatus of the preceding invention, a leukocyte-containing fluid is preferably filtered at a rate of less than 100 g/min. and not less than 20 g/min.
The leukocyte-removing filter apparatus of the preceding invention can be used not only for removing leukocytes responsible for various adverse side effects after blood transfusion using a blood product for transfusion, but also for removing leukocytes in an extracorporeal circulation therapy for autoimmune diseases. The extracorporeal circulation therapy for autoimmune diseases comprises filtering the leukocyte-containing body fluid of a patient continuously by the use of the leukocyte-removing filter apparatus of the preceding invention, returning the recovered fluid into the body, and thereby removing leukocytes from the body fluid.